1. Field of the Invention
The present invention relates to a method and a device for forming an ultrafine particle film of compound by evaporating a material within a reactive gas of low pressure and making the evaporated material and the reactive gas interact with each other.
2. Description of the Prior Art
Ultrafine particles of compound can be formed by heating a material within a vacuum vessel to evaporate the material, and introducing a reactive gas into the vacuum vessel to make the evaporated material and the reactive gas interact with each other.
For example, conventionally, a film of ultrafine particles has been formed by evaporating tin(Sn), making the evaporated tin and oxygen (O.sub.2) interact with each other to obtain ultrafine particles of tin dioxide(SnO.sub.2) and making the obtained ultrafine particles of tin dioxide deposit on the surface of a base plate provided in the vacuum vessel. This film is used as a gas sensing element of a gas sensor.
However, the above described method has a problem that the evaporated material and the reactive gas does not completely react to each other and incompletely reacted ultrafine particles are obtained as by-products.
For example, when ultrafine particles of SnO.sub.2 is formed, ultrafine particles of tin monoxide(SnO) are obtained as by-products due to the incomplete reaction of Sn and O.sub.2.
For obtaining the ultrafine particles of SnO.sub.2 without containing SnO, a method of heating ultrafine particles of a mixture of formed SnO.sub.2 and SnO in an atmosphere containing oxygen to transform SnO to SnO.sub.2 has been proposed. However, this method has such a problem that the producing steps are increased and that the ultrafine particles grow to a large size due to the above heat treatment. Furthermore, this method has another problem that the quality of the base plate is lowered because of the heat treatment. In addition, when this method is used in forming a gas sensor wherein a film of ultrafine particles is formed on a base plate provided with circuits, the reliability of the obtained gas sensor is in danger of being lowered due to the heat treatment.
Another method of making Sn and O.sub.2 excited in a high-frequency electric field interact with each other has been also proposed for improving the reactivity of Sn and O.sub.2.
However, when Sn is evaporated in an atmosphere of O.sub.2 of 1 Torr according to this method, the maximum content of SnO.sub.2 in the formed ultrafine particles is 70% or less.